The present invention relates to a method of forming a waveguide lens having a predetermined distribution of refractive index.
Known waveguide lenses formed in a two-dimensional waveguide on a substrate having an optical integrated circuit formed thereon include: a mode index lens which is formed by diffusion of an impurity into a region 50 of the surface of a substrate 10, which region 50 has a configuration similar to the cross sectional shape of an ordinary convex lens, as indicated in FIG. 6; a geodesic lens which is produced by first forming a small part-spherical concavity 52 in the surface of the substrate 10, and then forming a two-dimensional waveguide 12 over the entire surface of the substrate 10, as indicated in FIG. 7; and a fresnel lens which is formed by diffusion of an impurity into a region 54 of a surface of the substrate 10, which region 54 has a shape similar to the cross sectional shape of an ordinary fresnel lens, as indicated in FIG. 8. However, the above types of lenses have respective drawbacks. More specifically, with the mode index lens it is inherently difficult to obtain a sufficiently small F number, and the geodesic lens suffers from difficulty in forming the part-spherical concavity 52. Further, the fresnel lens tends to have a high degree of scattering of an incident light.
On the other hand, it is proposed to form a waveguide lens on a substrate, as shown in FIG. 1, so that the refractive index of the lens continuously changes in a direction intersecting a direction of propagation of a light beam. The waveguide lens having a refractive index distribution thus formed exhibits comparatively excellent optical properties, even if the size of the lens is relatively small, and is therefore free of the drawbacks as encountered with the known types of lens discussed above.
A conventional method for producing such a waveguide lens having a refractive index distribution requires a step of forming a thin film 56 of a suitable diffusion metal which is deposited on the surface of the substrate 12, for diffusion of the metal into the substrate in a subsequent step. However, as shown in FIG. 9 by way of example, the diffusion metal film 56 must have a continuously varying thickness, so that the density of the metal diffusion within the substrate is continuously changed. Since the thickness of the metal film 56 is inherently small, it is difficult to accurately control the thickness of the thin film 56 so that a variation of the thickness exactly corresponds to the desired distribution of refractive index of the waveguide lens to be obtained. Therefore, the above method has been found difficult to practice for large-scale production of such waveguide lenses without a fluctuation in the refractive index distribution from one lens to another.